1. Field of the Invention
The present invention relates to an apparatus and method for injection molding hollow shaped bodies by means of a short-shot method.
2. Description of Related Art
Various kinds of hollow molded products have been increasingly manufactured for the purpose of saving resources and reducing weight. In manufacturing these hollow molded products, a short-shot method has been conventionally employed, as disclosed in Japanese Patent Application Laid Open No. 56832/1981 and U.S. Pat. No. 4,101,617. According to this method, resin is preliminary injected into a cavity so as to be 50 to 90% of its capacity. Fluid, such as gas, is then injected into the resin for producing a hollow molded product. Prior to injection, the resin is heated to be in a hot molten state. The resin remains soft even while the fluid is injected. After the fluid is injected, a mold for hollow injection molding is cooled, by which, the resin therein hardens.
Hollow molded products produced in the above way may be used for example as an assist grip mountable in a vehicle interior. See grip 9 shown in FIG. 10. The assist grip 9 is provided with bent portions 91 on its right and left sides, each end portion having a space 92 at its end for accommodating fixtures. The assist grip 9 is composed of a resin part 39 as an outer shell, and a hollow section 49.
In the short-shot method, the resin is injected halfway into the cavity and forced forward by the fluid injected thereafter. In the above method, a hollow section is formed within the body. However, when producing the hollow molded product such as the assist grip 9 having bent portions 91 according to this method, an in-course 90 is likely to form a thin wall 99, as shown in FIG. 11. This is because the fluid injected into the resin tends to flow along the in-course 90 where the flow resistance is relatively low, resulting in a thin outer shell.
The thus-formed thin wall 99 decreases in strength and is easily cracked, thus causing defects. The resin around the thin wall 99 that has been somewhat cooled will flow abruptly due to injected fluid, resulting in albinism formed in the thin wall. This phenomenon deteriorates the appearance of the bent portion 91 of the assist grip 9. In order to fit a screw into a fixture space of the hollow section, it is preferable to have a thick wall around the bent portion. As described above, the hollow molded product is required to have an outer shell as a solid section made partially thick.